Use nsdialogs:
http://nsis.sourceforge.net/Docs/nsD...ctfolderdialog
If you haven't used nsdialogs before it'd be a bit difficult to get it. You have to create the label, text box and a browse button and when the browse button is click, display a selectfolderdialog, get what was selected and send it back to the text box. On the leave page you store the value of the text box in a variable. It is easier to use a program like ResEdit to design the form (140x300 in dialog units for MUI2) to get the x y width & height values. But review the nsidalogs readme to get the nsdialogs basics down..
Here is an attached working example.. you can compile it without any extra files.